Adhesive materials have been used for the restoration of wet bodies such as biological hard tissues like teeth and bones. As the adhesive materials for the wet bodies, resin-based curable compositions including a radical polymerizable monomer and a polymerization initiator have been widely used.
It is discussed to have the resin-based curable compositions include a radical polymerizable monomer having an acidic group and a polymerization initiator that cures highly efficiently the curable compositions containing the radical polymerizable monomer, in order to enhance the adhesive properties of the curable compositions to a wet body, particularly to biological hard tissues.
To bond a resin-based curable composition to a wet body, particularly to a tooth structure, it is necessary to allow the curable composition to penetrate into the tooth structure. A polymerizable monomer having a hydrophilic group and a polymerizable monomer having an acidic group in the curable composition penetrate into the tooth structure. When these components penetrate into the tooth structure, a curing reaction proceeds also inside the tooth structure near the interface between the tooth structure and the curable composition, forming a complex body of the curable composition and the tooth structure, that is, a resin-impregnated layer, thereby exhibiting a high bond strength.
The present applicants have proposed redox-curing type compositions each including: a radical polymerizable monomer; and an oxidizing agent and a reducing agent forming a redox polymerization initiator. As the reducing agent, a water-soluble reducing agent is dispersed in a powdery state in each redox-curing type composition so that the curing reaction of the composition is accelerated at an adhesion interface by the moisture in a wet body. For example, Patent Literature 1 proposes a redox curing-type nonaqueous curable composition including a liquid radical polymerizable monomer, an organic peroxide, and a powdery water-soluble reducing compound. The powdery water-soluble reducing compound is dispersed in the liquid radical polymerizable monomer. Patent Literature 2 proposes a multi-part redox-curing type composition including: a first part in which at least an oxidizing agent is dissolved in a first radical polymerizable monomer containing a radical polymerizable monomer having an acidic group and/or a hydrophilic group; and a second part in which at least an aromatic sulfinate is dispersed in a second radical polymerizable monomer containing a radical polymerizable monomer having neither an acidic group nor a hydrophilic group. These redox-curing type compositions achieved higher bond strengths to tooth structure, particularly dentin than those of conventional ones.
However, in Patent Literature 1, since a water-insoluble organic peroxide is used as the oxidizing agent of the redox polymerization initiator, it fails to penetrate sufficiently into a tooth structure, particularly into dentin, and fails to contribute sufficiently to the curing reaction inside the tooth structure. Thus, the bond strength is left to be improved.
Also in Patent Literature 2, since the oxidizing agent studied is a water-insoluble organic peroxide, it fails to penetrate sufficiently into a tooth structure, particularly into dentin, and fails to contribute sufficiently to the curing reaction inside the tooth structure. Thus, the bond strength is left to be improved.
In contrast, Patent Literature 3 discloses a dental cement including: a first paste containing a specified (meth)acrylate monomer, a filler, and an amine compound; and a second paste containing a specified (meth)acrylate monomer, a filler, an organic aromatic compound containing at least one —SO2— group as a polymerization catalyst, and a peroxide. More specifically, in this dental cement, the second paste contains a water-soluble inorganic peroxide serving as an oxidizing agent and the first paste contains water so that the inorganic peroxide is dissolved and accelerates the curing reaction when these pastes are mixed with each other. However, the bond strength of the dental cement is in a range equivalent to the ranges of the bond strengths of conventional dental cements. Moreover, there is a problem in that the water contained in the paste lowers the storage stability of the paste, and furthermore, the volatilization of the water during storage changes the composition of the paste, leading to possible variations in the curing time and curing property. An attempt to suppress the water evaporation requires complicated storage conditions, which is another problem. Moreover, when the composition contains water, a hydrophilic component and a hydrophobic component are separated from each other after the cement is prepared by mixing, and a cured product of the cement is degraded because the hydrophilic component absorbs water or is dissolved when immersed in water for a long period of time. This causes problems such as decreases in the bond durability, strength and transparency, and a change in color tone.